SMTP authentication problems continue

ZCS 4.5.9_GA_1454.UBUNTU6 (OS) on Ubuntu 7.10

I'm still trying to set things up so that authenticated users (that is, users who can log in via the Zimbra web interface without problem) can send emails via the Zimbra mail server using Thunderbird or Outlook from an outside-the-network location to an outside-the-network address.

Note that, sure enough, there is NO smtpd.crt in /opt/zimbra/conf. Is that the key to the problem? How do I rectify the situation?

With Thunderbird set for SSL for both incoming and outgoing, it generates the following error message:

The message could not be sent because connecting to SMTP server mail.imacs.org failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator.

Hi at all.
i've got the same problem in the same configuration of
zimbraMtaAuthEnabled: TRUE
zimbraMtaAuthURL: http://****/service/soap/
zimbraMtaTlsAuthOnly: TRUE
zimbraMailMode: http
And i'm not able to send email outside of the lan (from my pda with 3g connection for example).
Any suggestion?
because of i do not understand uscell answer
tnx in advance